The level of fluid in the borehole of a well is an important element in the field of oil well testing and operation. Determination of the liquid level in a well by an acoustic pulse has been successfully performed for many years. For example, U.S. Pat. No. 2,232,476 issued to Ritzmann in 1941, discloses the basic methodology wherein a high frequency acoustic pulse, generated by an acoustic pulse gun, is projected down the annulus between the tubing string and the well casing string. Acoustic pulse generating means typically include employing a blank shotgun shell to generate the sonic event, discharging a compressed gas into the annulus to generate the sonic event, or in a wellbore which has a substantial gas pressure it is possible to use the gas pressure itself to create the sonic event. Reflections of the sonic event or acoustic pulse are generated by cross-sectional variations along the length of the tubing string, such as are created by tubing collars or the surface of the liquid column in the annulus. A microphone or acoustic transducer is then used to sense the reflections and provide a signal indicative of those reflections.
The depth to the liquid surface is then determined by counting the number of tubing collars above the liquid interface and multiplying by the average distance between the collars, as indicated by the reflections. Interpretation of the reflection results is typically assisted by passing the electrical output signal of the acoustic transducer of the pulse gun through: 1) a high-pass or band pass filter so as to emphasize the short, high-frequency reflections from tubing collars; and/or 2) a low-pass filter to emphasize the low-frequency pulse from the gas-liquid interface.
As shown in FIG. 1, modern day pulse gun systems often comprise a pulse gun and microphone assembly connected to a computer or recorder device by means of a lead, cable or similar conductor. Analog signals produced by the microphone or acoustic transducer inside the gun assembly are transmitted along the length of cable and fed to the computer or recording device. Typically the recording device contains electronics to subsequently amplify, filter and then digitize the analog signals received from a cable or conductor. For example, see U.S. Pat. No. 5,117,399 to McCoy et. al where an electronics module contains the electronics to amplify, filter and digitize the analog signals from a microphone cable.
The digitization of the transducer's signal allows for additional analysis of the signal and also for digital storage of the signal. Additionally, various schemes for computer aided interpretation of the reflections are taught by the prior art, see for example U.S. Pat. No. 4,318,298, issued to Godbey, U.S. Pat. No. 4,793,178, issued to Ahern, U.S. Pat. No. 5,200,894, issued to McCoy and Canada Pat. No. 2,201,215 to Grande.
Accordingly, a necessary component of the acoustic pulse gun system is the means for reliably transmitting the signals produced by the transducer inside the pulse gun, which are indicative of the acoustic reflections, to the computer or recording device. However, state-of-the-art cables have relatively poor transmission qualities and the traditional cable system is sensitive to static and electronic noise which may be picked up along the length of a cable. The cable often acts as an antennae picking up electronic noise from a variety of sources. Typical sources of such noise in a well operation environment include electric motors operating nearby, the cathotic protection system installed to protect the metal elements of a well from deteriorating, and cell phones used by personnel. Noise and other extraneous signals can also be produced if the cable is moved during operations; such as due to wind or by an operator accidentally bumping it.
What is desired therefore is a system or method which overcomes the analog transmission difficulties and provides reliable transmission of the signals from a pulse gun assembly to a computer or recorder.